Tires for conventional sized automobiles are mounted on metal wheel rims that are essentially of one piece construction. The beads at the inner perimeter of the rubber tire are flexible and stretchable to the extent that they can be slightly deformed so as to pass over the side flanges of the wheel rim and become seated on the rim between the side flanges.
When the size of the rubber tire is increased, the strength of the beads is also increased, usually with the placement of more or stronger steel cords that extend annularly within the beads of the tire. Because of this increased strength capacity, it is much more difficult to stretch and/or deform the beads of large and stronger tires in order to place the beads on a unitary rim structure.
To permit ease of mounting and demounting tires on wheel rim structures, a multiple piece steel rim structure was developed which could be assembled as the tire is being mounted on the rim structure. Typically, the multiple piece rim structure is a five piece rim structure which includes a cylindrical rim base of unitary construction with opposed annular edges. A separately formed back side ring is telescopically mounted on the rim base and held in place on the back section of the rim base by a protruding back flange of the rim base. The tire is then mounted telescopically about the rim base with its back bead engaging the back side ring, and a front side ring is placed about the rim base behind the tire, so that it engages the front bead of the tire. An annular bead seat is telescopically inserted through the front side ring and the front bead of the tire and about the front edge of the rim base, and is held in place by a lock ring which engages a lock ring recess in the rim base. With the tire and the wheel rim assembled in this manner, the tire is inflated and the wheel assembly is ready for mounting to a vehicle.
One of the major problems with the above noted five piece wheel rim when used on a large industrial vehicle which handles heavy loads under stressful circumstances is that there are many instances where the vehicle rapidly accelerates or decelerates and the tire having traction with the ground, resists spinning or skidding with respect to the ground while the wheel rim responds to the drive train or brakes of the vehicle in an attempt to force the tire to rotate or to stop its rotation. This results in relative movement of the rubber tire with respect to the steel wheel rim. More particularly, the frictional engagement between the inner side ring and the rubber tire is sufficient to cause the inner side ring to move in unison with the tire while the rim base moves either faster or slower than the tire and the inner side ring. This causes relative movement between the inner side ring and the rim base, and the result is fretting or deterioration of the steel of the facing surfaces of the inner side ring and the rim base. Over time, the fretting becomes so serious as to cause failure of the inner side ring and/or the rim base. The deterioration can cause air leakage between the parts, partially or complete separation of the parts, and injury to the equipment and to the personnel operating or adjacent the equipment.
The typical five piece steel wheel rim structures which are used for very large vehicles, such as earth movers and large dump trucks weighing 100 tons or more, have a cold tire pressure of over 1000 lb. psi. This tire pressure could easily rise to 1400 lb. psi as the air, tires and wheels heat during use of the vehicle. This tends to build up an enormous potential energy within the tires this size, and there is a hazard that the side flanges of the five piece rim structures could release under the force of the compressed air in the tire, creating extreme risk to people and equipment adjacent the wheel. The release of the side flange could be caused by cracks in the flange or by fretting of the flange which occurs at the facing surfaces of the side flange and rim base.
Moreover, large earth moving vehicles in use today typically run on radial ply tires. Radial tires offer the advantages of being able to handle greater loads at greater speeds. Radial tires have proven to be longer lasting, have better traction, have more even ground pressure which results in better floatation of the vehicle, a smoother ride and improved fuel economy. Radial tires under optimal use and conditions will last sometimes twice as long as bias ply tires.
The advantages of using radial ply tires over bias ply tires is the obvious reason why there has been such a large shift toward using radial tires. However, the use of radial ply tires has necessitated a redesign of the rims used with the tires because of the greater stresses applied to the side rings and back sections of the rim bases.
As illustrated in FIGS. 4 and 5 of the drawings, the forces that act upon a wheel rim are different with bias ply tires and radial ply tires. FIG. 4 illustrates a bias ply tire, and FIG. 5 illustrates a radial ply tire. The lines of force 101 and 102 that act upon the rim in a bias ply tire extend diagonally across the face of the tire. The construction of the tire is crisscrossed layers of tire ply material. The forces of stress induced in the tire generally follow these biased or angled threads of the plies which run at angles to one another. The net result is a stiffer tire side wall and the stresses of the tire at the bottom dead center of the tire are disbursed both radially and circumferentially about the tire at an angle over a wide area of the tire bead and wheel rim. There is less stress applied from the tire to the side rings, since the stress is spread about more of the side rings, and more stress placed on the tire bead mounting area of the rim.
FIG. 5 shows the lines of force 103 and 104 in a radial ply tire, which tend to follow the radially extending cords of the radial plies. The net result is that the lines of force run more radially and less circumferentially and do not criss-cross as much as in the bias ply tire. The result is a softer side wall of the radial ply tire. The stress is concentrated more on the side rings at bottom dead center of the wheel than about the rim and tire bead area. The stress induced on the side rings tends to cause them to flex radially like the tire does. Since the side rings are anchored by the back section interface area, the rim back section area will act as a fulcrum for a side ring under stress that is trying to flex in a radial direction. The net result is significantly greater stress concentrated on smaller areas of side rings and the rim back section interface.
Radial tires mounted on a rim can exert static loads on the rim up to 250% greater to than that of a bias ply tire. This increased load stress can eventually cause rims to fail prematurely. The greatest increase in stress is on the rim's side rings and back section. These stresses can shorten the rim life by as much as 50% to 75%. Previous rim design changes have added more material thickness to the side rings and back section of the rim base. The direct result of these changes was increased rim life. However, two problems were not solved with these changes: rim fretting and rim fatigue.
Fretting is the result of two things. The first cause is radial stress from using radial tires. The second cause is from the side ring "walking" around the rim, as previously described. This results from: the rotation of the rim or the wheel, the side ring not being fixed to the rim base, and the tires tendency to move minute amounts about the rim base during acceleration and deceleration of the vehicle. The air pressure in the tire exerts thousands of pounds per square inch of pressure and force on the side ring. The side ring is, as a result, pushed against the back flange interface area. The back flange, being relatively fixed, exerts force back on the side ring. When the tire begins to "walk" circumferentially on the rim, the side ring moves with the tire, not with the rim base. It is not uncommon to have a tire and side flange walk a distance of ten inches in an eight hour shift in haul truck applications at a mine site. At the point where the side flange and the rim back section interface, fretting will begin to take place in minute amounts. It is the net accumulation of this fretting that leads to the eventual wear on the rim back flange. If left unchecked, this wear can result in cracks in the rim and excessive wear and premature failure of the side rings. All of these problems result in increased operating costs to the end user, which is something all end users are interested in reducing.
The problem of the side ring walking about the rim base has not been solved. Attempts have been made to stop the flange from walking by using "flange locks." These locks are welded on the rim back section along with a mating lock welded onto the side flange. This has proven to be unsuccessful. In the applications tested, the net result has been that some of the flange locks were literally torn off the rim back flange because of the circumferential forces applied by the tire to the rim. In some cases, the rim back flanges were under such stress that the area surrounding the flange locks developed fractures and failure occurred in that sections of the rim back flange were sheared off. In other instances, the flange locks welded to the side rings also sheared off as a result of the stress placed on them.
Repair of the rim base involves cutting or machining off the rim back flange and replacing it with a new flange. Repair of the rim base is only an option if the rim is in otherwise good condition.
Another problem that occurs is side ring wear. The side ring wear is the result of abrasive aggregates getting down between the side ring and the tire. Over time, this aggregate grinds away at the side ring and causes a thinning of the ring material. This is an accepted problem and is a reason so many of the components are replaced. Side rings are a minimal cost when compared to the cost of a rim base or entire wheel.